Download Chapter 33 Invertebrates

Chapter 33
Invertebrates
Lecture Outline
Overview: Life Without a Backbone
 Invertebrates—animals without a backbone—account for 95%
of known animal species and all but one of the roughly 35
animal phyla that have been described.
 More than a million extant species of animals are known, and
at least as many more will probably be identified by future
biologists.
 Invertebrates inhabit nearly all environments on Earth, from
the scalding water of deep-sea hydrothermal vents to the
rocky, frozen ground of Antarctica.
Concept 33.1 Sponges are sessile and have a porous body and
choanocytes
 Sponges (phylum Porifera) are so sedentary that they were
mistaken for plants by the early Greeks.
 Living in freshwater and marine environments, sponges are
suspension feeders.
 The body of a simple sponge resembles a sac perforated with
holes.
 Water is drawn through the pores into a central cavity, the
spongocoel, and flows out through a larger opening, the
osculum.
 More complex sponges have folded body walls, and many
contain branched water canals and several oscula.
 Sponges range in height from about a few mm to 2 m and most
are marine.
 About 100 species live in fresh water.
 Unlike eumetazoa, sponges lack true issues, groups of similar
cells that form a functional unit.
 The germ layers of sponges are loose federations of cells,
which are not really tissues because the cells are relatively
unspecialized.
 The sponge body does contain different cell types.
 Sponges collect food particles from water passing through
food-trapping equipment.
 Flagellated choanocytes, or collar cells, lining the
spongocoel (internal water chambers) create a flow of water
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through the sponge with their flagella and trap food with
their collars.
 Based on both molecular evidence and the morphology of
their choanocytes, sponges evolved from a colonial
choanoflagellate ancestor.
The body of a sponge consists of two cell layers separated by a
gelatinous region, the mesohyl.
Wandering though the mesohyl are amoebocytes.
 They take up food from water and from choanocytes, digest
it, and carry nutrients to other cells.
 They also secrete tough skeletal fibers within the mesohyl.
 In some groups of sponges, these fibers are sharp
spicules of calcium carbonate or silica.
 Other sponges produce more flexible fibers from a
collagen protein called spongin.
 We use these pliant, honeycombed skeletons as bath
sponges.
Most sponges are sequential hermaphrodites, with each
individual producing both sperm and eggs in sequence.
 Gametes arise from choanocytes or amoebocytes.
 The eggs are retained, but sperm are carried out the
osculum by the water current.
 Sperm are drawn into neighboring individuals and fertilize
eggs in the mesohyl.
 The zygotes develop into flagellated, swimming larvae that
disperse from the parent.
 When a larva finds a suitable substratum, it develops into a
sessile adult.
Sponges produce a variety of antibiotics and other defensive
compounds.
 Researchers are now isolating these compounds, which may
be useful in fighting human disease.
Concept 33.2 Cnidarians have radial symmetry, a
gastrovascular cavity, and cnidocytes
 All animals except sponges belong to the Eumetazoa, the
animals with true tissues.
 The cnidarians (hydras, jellies, sea anemones, and coral
animals) have a relatively simple body construction.
 They are a diverse group with more than 10,000 living
species, most of which are marine.
 They exhibit a relatively simple, diploblastic body plan that
arose 570 million years ago.
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 The basic cnidarian body plan is a sac with a central digestive
compartment, the gastrovascular cavity.
 A single opening to this cavity functions as both mouth and
anus.
 This basic body plan has two variations: the sessile polyp and
the floating medusa.
 The cylindrical polyps, such as hydras and sea anemones,
adhere to the substratum by the aboral end and extend their
tentacles, waiting for prey.
 Medusas (also called jellies) are flattened, mouth-down
versions of polyps that move by drifting passively and by
contracting their bell-shaped bodies.
 The tentacles of a jelly dangle from the oral surface.
 Some cnidarians exist only as polyps.
 Others exist only as medusas.
 Still others pass sequentially through both a medusa stage
and a polyp stage in their life cycle.
 Cnidarians are carnivores that use tentacles arranged in a ring
around the mouth to capture prey and push the food into the
gastrovascular chamber for digestion.
 Batteries of cnidocytes on the tentacles defend the animal
or capture prey.
 Organelles called cnidae evert a thread that can inject
poison into the prey, or stick to or entangle the target.
 Cnidae called nematocysts are stinging capsules.
 Muscles and nerves exist in their simplest forms in cnidarians.
 Cells of the epidermis and gastrodermis have bundles of
microfilaments arranged into contractile fibers.
 True muscle tissue appears first in triploblastic animals.
 When the animal closes its mouth, the gastrovascular cavity
acts as a hydrostatic skeleton against which the contractile
cells can work.
 Movements are controlled by a noncentralized nerve net
associated with simple sensory receptors that are distributed
radially around the body.
 The phylum Cnidaria is divided into four major classes:
Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa.
 The four cnidarian classes show variations on the same body
theme of polyp and medusa.
 Most hydrozoans alternate polyp and medusa forms, as in the
life cycle of Obelia.
 The polyp stage, often a colony of interconnected polyps, is
more conspicuous than the medusa.
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 Hydras, among the few freshwater cnidarians, are unusual
members of the class Hydrozoa in that they exist only in the
polyp form.
 When environmental conditions are favorable, a hydra
reproduces asexually by budding, the formation of
outgrowths that pinch off from the parent to live
independently.
 When environmental conditions deteriorate, hydras form
resistant zygotes that remain dormant until conditions
improve.
 The medusa generally prevails in the life cycle of class
Scyphozoa.
 The medusae of most species live among the plankton as
jellies.
 Most coastal scyphozoans go through small polyp stages during
their life cycle.
 Jellies that live in the open ocean generally lack the sessile
polyp.
 Cubozoans have a box-shaped medusa stage.
 They can be distinguished from scyphozoans in other
significant ways, such as having complex eyes in the fringe
of the medusae.
 Cubozoans, which generally live in tropical oceans, are often
equipped with highly toxic cnidocytes.
 Sea anemones and corals belong to the class Anthozoa.
 They occur only as polyps.
 Coral animals live as solitary or colonial forms and secrete a
hard external skeleton of calcium carbonate.
 Each polyp generation builds on the skeletal remains of
earlier generations to form skeletons that we call coral.
 In tropical seas, coral reefs provide habitat for a great
diversity of invertebrates and fishes.
 Coral reefs in many parts of the world are currently being
destroyed by human activity.
 Pollution, overfishing, and global warming are contributing to
their demise.
Concept 33.3 Most animals have bilateral symmetry
 The vast majority of animal species belong to the clade
Bilateria, which consists of animals with bilateral symmetry
and triploblastic development.
 Most bilaterians are also coelomates.
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 The most recent common ancestor of living bilaterians
probably lived in the later Proterozoic.
 During the Cambrian explosion, most major groups of
bilaterians emerged.
Phylum Platyhelminthes: Flatworms are acoelomates with
gastrovascular cavities.
 Flatworms live in marine, freshwater, and damp terrestrial
habitats.
 They also include many parasitic species, such as the flukes
and tapeworms.
 Flatworms have thin bodies, ranging in size from nearly
microscopic to tapeworms more than 20 m long.
 Flatworms and other bilaterians are triploblastic, with a middle
embryonic tissue layer, a mesoderm, which contributes to more
complex organs and organ systems and to true muscle tissue.
 While flatworms are structurally more complex than
cnidarians, they are simpler than other bilaterians.
 Like cnidarians, flatworms have a gastrovascular cavity with
only one opening (and tapeworms lack a digestive system
entirely and absorb nutrients across their body surface).
 Unlike other bilaterians, flatworms lack a coelom.
 The flat shape of a flatworm places all cells close to the
surrounding water, enabling gas exchange and the elimination
of nitrogenous wastes (ammonia) by diffusion across the body
surface.
 Flatworms have no specialized organs for gas exchange and
circulation, and their relatively simple excretory apparatus
functions mainly to maintain osmotic balance.
 This apparatus consists of ciliated cells called flame bulbs
that waft fluid through branched ducts that open to the
outside.
 Flatworms are divided into four classes: Turbellaria,
Monogenia, Trematoda, and Cestoidea.
 Turbellarians are nearly all free-living (nonparasitic) and most
are marine.
 Planarians, members of the genus Dugesia, are carnivores
or scavengers in unpolluted ponds and streams.
 Planarians move using cilia on the ventral epidermis, gliding
along a film of mucus they secrete.
 Some turbellarians use muscles for undulatory swimming.
 A planarian has a head with a pair of eyespots to detect light,
and lateral flaps that function mainly for smell.
 The planarian nervous system is more complex and centralized
than the nerve net of cnidarians.
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 Planarians can learn to modify their responses to stimuli.
Planarians reproduce asexually through regeneration.
 The parent constricts in the middle, and each half
regenerates the missing end.
Planarians can also reproduce sexually.
 These hermaphrodites cross-fertilize.
The monogeneans (class Monogenea) and the trematodes (class
Trematoda) live as parasites in or on other animals.
 Many have suckers for attachment to their host.
 A tough covering protects the parasites.
 Reproductive organs nearly fill the interior of these worms.
Trematodes parasitize a wide range of hosts, and most species
have complex life cycles with alternation of sexual and asexual
stages.
 Many require an intermediate host in which the larvae
develop before infecting the final hosts (usually a
vertebrate) where the adult worm lives.
 The blood fluke Schistosoma infects 200 million people,
leading to body pains and dysentery.
 The intermediate host for Schistosoma is a snail.
Living within different hosts puts demands on trematodes that
free-living animals do not face.
 A blood fluke must evade the immune systems of two very
different hosts.
 By mimicking their host’s surface proteins, blood flukes
create a partial immunological camouflage.
 They also release molecules that manipulate the host’s
immune system.
 These defenses are so effective that individual flukes can
survive in a human host for more than 40 years.
Most monogeneans are external parasites of fishes.
 Their life cycles are simple, with a ciliated, free-living larva
that starts an infection on a host.
 While traditionally aligned with trematodes, some structural
and chemical evidence suggests that they are more closely
related to tapeworms.
Tapeworms (class Cestoidea) are also parasitic.
 The adults live mostly in vertebrates, including humans.
Suckers and hooks on the head, or scolex, anchor the worm in
the digestive tract of the host.
 Tapeworms lack a gastrovascular cavity and absorb food
particles from their hosts.
A long series of proglottids, sacs of sex organs, lie posterior
to the scolex.
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 Mature proglottids, loaded with thousands of eggs, are
released from the posterior end of the tapeworm and leave
with the host’s feces.
 In one type of cycle, tapeworm eggs in contaminated food or
water are ingested by intermediary hosts, such as pigs or
cattle.
 The eggs develop into larvae that encyst in the muscles of
their host.
 Humans acquire the larvae by eating undercooked meat
contaminated with cysts.
 The larvae develop into mature adults within the human.
Phylum Rotifera: Rotifers are pseudocoelomates with jaws,
crowns of cilia, and complete digestive tracts.
 Rotifers are tiny animals (5 µm to 2 mm), most of which live in
freshwater.
 Some live in the sea or in damp soil.
 Rotifers are smaller than many protists but are truly
multicellular, with specialized organ systems.
 Rotifers have an alimentary canal, a digestive tract with a
separate mouth and anus.
 Internal organs lie in the pseudocoelom, a body cavity that is
not completely lined with mesoderm.
 The fluid in the pseudocoelom serves as a hydrostatic
skeleton.
 Through the movements of nutrients and wastes dissolved in
the coelomic fluid, the pseudocoelom also functions as a
circulatory system.
 The word rotifer, “wheel-bearer,” refers to the crown of cilia
that draws a vortex of water into the mouth.
 Food particles drawn in by the cilia are captured by the
jaws (trophi) in the pharynx and ground up.
 Some rotifers exist only as females that produce more
females from unfertilized eggs, a type of parthenogenesis.
 Other species produce two types of eggs that develop by
parthenogenesis.
 One type forms females, and the other forms degenerate
males that survive just long enough to fertilize eggs.
 The zygote forms a resistant stage that can withstand
environmental extremes until conditions improve.
 The zygote then begins a new female generation that
reproduces by parthenogenesis until conditions become
unfavorable again.
 It is puzzling that so many rotifers survive without males.
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 The vast majority of animals and plants reproduce sexually
at least some of the time, and sexual reproduction has
certain advantages over asexual reproduction.
 For example, species that reproduce asexually tend to
accumulate harmful mutations in their genomes faster than
sexually reproducing species.
 As a result, asexual species experience higher rates of
extinction and lower rates of speciation.
 A class of asexual rotifers called Bdelloidea consists of 360
species that all reproduce by parthenogenesis without males.
 Thirty-five-million-year-old bdelloid rotifers have been
found preserved in amber.
 The morphology of these fossils resembles the female form.
 DNA comparisons of bdelloids with their closest sexually
reproducing rotifer relatives suggest that bdelloids have
been asexual for far more than 35 million years.
 Bdelloid rotifers raise interesting questions about the
evolution of sex.
The lophophorate phyla: ectoprocts, phoronids, and
brachiopods are coelomates with ciliated tentacles around
their mouths.
 Bilaterians in three phyla—Ectoprocta, Phoronida, and
Brachiopoda—are traditionally called lophophorate animals
because they all have a lophophore.
 The lophophore is a horseshoe-shaped or circular fold of
the body wall bearing ciliated tentacles that surround and
draw water toward the mouth.
 The tentacles trap suspended food particles.
 In addition to the lophophore, these three phyla share a Ushaped digestive tract and the absence of a head.
 These may be adaptations to a sessile existence.
 In contrast to flatworms, which lack a body cavity, and
rotifers, which have a pseudocoelom, lophophorates have true
coeloms completely lined with mesoderm.
 Ectoprocts are colonial animals that superficially resemble
plants.
 In most species, the colony is encased in a hard exoskeleton.
 The lophophores extend through pores in the exoskeleton.
 Most ectoprocts are marine, where they are widespread and
numerous sessile animals, with several species that can be
important reef builders.
 Ectoprocts also live in lakes and rivers.
 Phoronids are tube-dwelling marine worms ranging from 1 mm
to 50 cm in length.
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 Some live buried in the sand within chitinous tubes.
 They extend the lophophore from the tube when feeding
and pull it back in when threatened.
 Brachiopods, or lampshells, superficially resemble clams and
other bivalve molluscs.
 However, the two halves of the brachiopod are dorsal and
ventral to the animal, rather than lateral as in clams.
 All brachiopods are marine.
 Most live attached to the substratum by a stalk, opening
their shell slightly to allow water to flow over the
lophophore.
 The living brachiopods are remnants of a richer past.
 Thirty thousand species of brachiopod fossils have been
described from the Paleozoic and Mesozoic eras.
Phylum Nemertea: Proboscis worms are named for their
prey-capturing apparatus.
 The members of the Phylum Nemertea, proboscis worms or
ribbon worms, have bodies much like those of flatworms.
 However, they have a small fluid-filled sac that may be a
reduced version of a true coelom.
 The sac and fluid hydraulics operate an extensible
proboscis, which the worm uses to capture prey.
 Nemerteans range in length from less than 1 mm to several
meters.
 Nearly all nemerteans are marine, but a few species inhabit
fresh water or damp soil.
 Some are active swimmers, and others burrow into the sand.
 Nemerteans and flatworms have similar excretory, sensory,
and nervous systems.
 However, nemerteans have an alimentary canal and a closed
circulatory system in which the blood is contained in vessels.
 Nemerteans have no heart, and the blood is propelled by
muscles squeezing the vessels.
Concept 33.4 Molluscs have a muscular foot, a visceral mass,
and a mantle
 The phylum Mollusca includes many diverse forms, including
snails and slugs, oysters and clams, and octopuses and squids.
 Most molluscs are marine, though some inhabit fresh water,
and some snails and slugs live on land.
 Molluscs are soft-bodied animals, but most are protected by a
hard shell of calcium carbonate.
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 Slugs, squids, and octopuses have reduced or lost their
shells completely during their evolution.
Despite their apparent differences, all molluscs have a similar
body plan with a muscular foot (typically for locomotion), a
visceral mass with most of the internal organs, and a mantle.
 The mantle, which secretes the shell, drapes over the
visceral mass and creates a water-filled chamber, the
mantle cavity, with gills, anus, and excretory pores.
 Many molluscs feed by using a straplike rasping organ, a
radula, to scrape up food.
Most molluscs have separate sexes, with gonads located in the
visceral mass.
 However, many snails are hermaphrodites.
The life cycle of many marine molluscs includes a ciliated larva,
the trochophore.
 This larva is also found in marine annelids (segmented
worms) and some other lophotrochozoans.
The basic molluscan body plan has evolved in various ways in
the eight classes of the phylum.
 The four most prominent are the Polyplacophora (chitons),
Gastropoda (snails and slugs), Bivalvia (clams, oysters, and
other bivalves), and Cephalopoda (squids, octopuses,
cuttlefish, and chambered nautiluses).
Chitons are marine animals with oval shapes and shells divided
into eight dorsal plates.
 The chiton body is unsegmented.
Chitons use their muscular foot to grip the rocky substrate
tightly and to creep slowly over the rock surface.
Chitons are grazers that use their radulas to scrape and ingest
algae.
Almost three-quarters of all living species of molluscs are
gastropods.
 Most gastropods are marine, but there are also many
freshwater species.
 Garden snails and slugs have adapted to land.
During embryonic development, gastropods undergo torsion in
which the visceral mass is rotated up to 180 degrees, so the
anus and mantle cavity are above the head in adults.
 After torsion, some of the organs that were bilateral are
reduced or lost on one side of the body.
Most gastropods are protected by single, spiral shells into
which the animals can retreat if threatened.
 Torsion and formation of the coiled shell are independent
developmental processes.
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 While gastropod shells are typically conical, those of abalones
and limpets are somewhat flattened.
 Many gastropods have distinct heads with eyes at the tips of
tentacles.
 They move by a rippling motion of their foot or by means of
cilia.
 Most gastropods use their radula to graze on algae or plant
material.
 Some species are predators.
 In these species, the radula is modified to bore holes in the
shells of other organisms or to tear apart tough animal
tissues.
 In the tropical marine cone snails, teeth on the radula form
separate poison darts, which penetrate and stun their prey,
including fishes.
 In place of the gills found in most aquatic gastropods, the
lining of the mantle cavity of terrestrial snails functions as a
lung.
 The class Bivalvia includes clams, oysters, mussels, and
scallops.
 Bivalves have shells divided into two halves.
 The two parts are hinged at the mid-dorsal line, and
powerful adductor muscles close the shell tightly to protect
the animal.
 Bivalves have no distinct head, and the radula has been lost.
 Some bivalves have eyes and sensory tentacles along the
outer edge of the mantle.
 The mantle cavity of a bivalve contains gills that are used for
feeding and gas exchange.
 Most bivalves are suspension feeders, trapping fine particles in
mucus that coats the gills.
 Cilia convey the particles to the mouth.
 Water flows into the mantle cavity via the incurrent siphon,
passes over the gills, and exits via the excurrent siphon.
 Most bivalves live rather sedentary lives, a characteristic
suited to suspension feeding.
 Sessile mussels secrete strong threads that tether them to
rocks, docks, boats, and the shells of other animals.
 Clams can pull themselves into the sand or mud, using the
muscular foot as an anchor.
 Scallops can swim in short bursts to avoid predators by
flapping their shells and jetting water out their mantle
cavity.
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 Cephalopods are active predators that use rapid movements to
dart toward their prey, which they capture with several long
tentacles.
 Squids and octopuses use beak-like jaws to bite their prey
and then inject poison to immobilize the victim.
 A mantle covers the visceral mass, but the shell is reduced and
internal in squids, missing in many octopuses, and exists
externally only in chambered nautiluses.
 Fast movements by a squid occur when it contracts its mantle
cavity and fires a stream of water through the excurrent
siphon.
 By pointing the siphon in different directions, the squid can
rapidly move in different directions.
 The foot of a cephalopod has been modified into the muscular
siphon and parts of the tentacles and head.
 Cephalopods are the only molluscs with a closed circulatory
system.
 They also have well-developed sense organs and a complex
brain.
 The ancestors of octopuses and squid were probably shelled
molluscs that took up a predatory lifestyle.
 Shelled cephalopods called ammonites were the dominant
invertebrate predators of the seas for hundreds of millions of
years until their disappearance in the mass extinctions at the
end of the Cretaceous period.
 Most squid are less than 75 cm long.
 In 2003, a squid with a mantle 2.5 meters long was captured
near Antarctica.
 The specimen was possibly a juvenile, only half the size of
an adult.
 Large squid are thought to feed on large fish in the deep
ocean, where sperm whales are their only natural
predators.
Concept 33.5 Annelids are segmented worms
 All annelids (“little rings”) have segmented bodies.
 They range in length from less than 1 mm to 3 m for the giant
Australian earthworm.
 Annelids live in the sea, most freshwater habitats, and damp
soil.
 The phylum Annelida is divided into three classes: Oligochaeta
(earthworms), Polychaeta (polychaetes), and Hirudinea
(leeches).
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 Oligochaetes are named for their relatively sparse chaetae, or
bristles made of chitin.
 This class of segmented worms includes the earthworms and a
variety of aquatic species.
 Earthworms eat their way through soil, extracting nutrients as
the soil passes through the alimentary canal.
 Undigested material is egested as castings.
 Earthworms till the soil, enriching it with their castings.
 Earthworms are cross-fertilizing hermaphrodites.
 Two earthworms exchange sperm and then separate.
 The received sperm are stored while a special organ, the
clitellum, secretes a mucous cocoon.
 As the cocoon slides along the body, it picks up eggs and
stored sperm and slides off the body into the soil.
 Some earthworms can also reproduce asexually by
fragmentation followed by regeneration.
 Each segment of a polychaete (“many setae”) has a pair of
paddlelike or ridgelike parapodia (“almost feet”) that function
in locomotion.
 Each parapodium has several chitinous setae.
 In many polychaetes, the rich blood vessels in the parapodia
function as gills.
 Most polychaetes are marine.
 Many crawl on or burrow in the seafloor, while a few drift
and swim in the plankton.
 Some live in tubes that the worms make by mixing mucus
with sand and broken shells. Others construct tubes from
their own secretions.
 The majority of leeches inhabit fresh water, but land leeches
move through moist vegetation.
 Leeches range in size from about 1 to 30 cm.
 Many leeches feed on other invertebrates, but some bloodsucking parasites feed by attaching temporarily to other
animals, including humans.
 Some parasitic species use blade-like jaws to slit the host’s
skin, while others secrete enzymes that digest a hole
through the skin.
 The host is usually unaware of the attack because the leech
secretes an anesthetic.
 The leech also secretes hirudin, an anticoagulant, into the
wound, allowing the leech to suck as much blood as it can
hold.
 Until the 20th century, leeches were frequently used by
physicians for bloodletting.
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 Leeches are still used to drain blood that accumulates in
tissues following injury or surgery.
 Researchers are also investigating the potential use of
hirudin to dissolve unwanted blood clots from surgery or
heart disease.
 A recombinant form of hirudin has been developed and is in
clinical trials.
Concept 33.6 Nematodes are nonsegmented pseudocoelomates
covered by a tough cuticle
 Roundworms are found in most aquatic habitats, wet soil, moist
tissues of plants, and the body fluids and tissues of animals.
 They range in size from less than 1 mm to more than a meter.
 The cylindrical bodies of roundworms are covered with a tough
exoskeleton, the cuticle.
 As the worm grows, it periodically sheds its old cuticle and
secretes a new, larger one.
 They have an alimentary tract and use the fluid in their
pseudocoelom to transport nutrients since they lack a
circulatory system.
 Their thrashing motion is due to contraction of longitudinal
muscles.
 Nematodes usually reproduce sexually.
 The sexes are separate in most species, and fertilization is
internal.
 Females may lay 100,000 or more fertilized eggs per day.
 The zygotes of most nematodes are resistant cells that can
survive harsh conditions.
 Abundant, free-living nematodes live in moist soil and in
decomposing organic matter on the bottom of lakes and oceans.
 There are 25,000 described species, and perhaps ten times
that number actually exist.
 If nothing but nematodes remained, it has been said, they
would still preserve the outline of the planet and many of its
features.
 They play a major role in decomposition and nutrient
recycling.
 The soil nematode, Caenorhabditis elegans, has become a
model organism in developmental biology.
 The nematodes include many species that are important
agricultural pests that attack plant roots.
 Other species parasitize animals.
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 More than 50 nematode species, including various pinworms
and hookworms, parasitize humans.
 Trichinella spiralis causes trichinosis when the nematode
worms encyst in a variety of human organs, including
skeletal muscle.
 They are acquired by eating undercooked meat that has
juvenile worms encysted in the muscle tissue.
 Parasitic nematodes are able to hijack some of the cellular
functions of their hosts.
 Plant-parasitic nematodes produce molecules that induce
the development of root cells that provide nutrients to the
parasites.
 Trichenella in human muscle cells controls the expression of
muscle cell genes that code for proteins that make the cell
elastic enough to house the nematode.
 The muscle cell also releases signals to attract blood
vessels, supplying the nematode with nutrients.
Concept 33.7 Arthropods are segmented coelomates that have
an exoskeleton and jointed appendages
 The world18arthropod population has been estimated at a billion
billion (10 ) individuals.
 Nearly a million arthropod species have been described.
 Two out of every three known species are arthropods.
 This phylum is represented in nearly all habitats in the
biosphere.
 On the criteria of species diversity, distribution, and sheer
numbers, arthropods must be regarded as the most successful
animal phylum.
 The diversity and success of arthropods are largely due to
three features: body segmentation, a hard exoskeleton, and
jointed appendages.
 Early arthropods such as the trilobites had pronounced
segmentation, but little variation in their appendages.
 Groups of segments and their appendages have become
specialized for a variety of functions, permitting efficient
division of labor among regions.
 The body of an arthropod is completely covered by the cuticle,
an exoskeleton constructed from layers of protein and chitin.
 The exoskeleton protects the animal and provides points of
attachment for the muscles that move appendages.
 It is thick and inflexible in some regions, such as crab claws,
and thin and flexible in others, such as joints.
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 The exoskeleton of arthropods is strong and relatively
impermeable to water.
 In order to grow, an arthropod must molt its old
exoskeleton and secrete a larger one, a process called
ecdysis that leaves the animal temporarily vulnerable to
predators and other dangers.
 The exoskeleton’s relative impermeability to water helped
prevent desiccation and provided support on land.
 Arthropods moved to land after the colonization of land by
plants and fungi.
 In 2004, an amateur fossil hunter found a 428-million-yearold fossil of a millipede. Fossilized arthropod tracks date
from 450 million years ago.
 Arthropods have well-developed sense organs, including eyes
for vision, olfactory receptors for smell, and antennae for
touch and smell.
 Most sense organs are located at the anterior end of the
animal, which shows extensive cephalization.
 Arthropods have an open circulatory system in which
hemolymph fluid is propelled by a heart through short arteries
into sinuses (the hemocoel) surrounding tissues and organs.
 Hemolymph returns to the heart through valved pores.
 The hemocoel is not a coelom; the true coelom is much
reduced in most arthropods.
 Open circulatory systems evolved convergently in
arthropods and molluscs.
 Arthropods have evolved a variety of specialized organs for
gas exchange.
 Most aquatic species have gills with thin, feathery
extensions that have an extensive surface area in contact
with water.
 Terrestrial arthropods generally have internal surfaces
specialized for gas exchange.
 For example, insects have tracheal systems, branched air
ducts leading into the interior from pores in the cuticle.
 Molecular systematics is suggesting new hypotheses about
arthropod relationships.
 Evidence shows that arthropods diverged early in their
history into four main evolutionary lineages: cheliceriformes
(sea spiders, horseshoe crabs, scorpions, ticks, spiders),
myriapods (centipedes and millipedes), hexapods (insects
and their wingless, six-legged relatives), and crustaceans
(crabs, lobsters, shrimps, barnacles, and many others).
 Cheliceriformes are named for their clawlike feeding
appendages, chelicerae, which serve as pincers or fangs.
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 Cheliceriformes have an anterior cephalothorax and a
posterior abdomen.
 They lack sensory antennae, and most have simple eyes
(eyes with a single lens).
 The earliest cheliceriformes were eurypterids, or water
scorpions, marine and freshwater predators that grew up to
3 m long.
 Modern marine cheliceriformes include the sea spiders
(pycnogonids) and the horseshoe crabs.
The majority of living cheliceriformes are arachnids, a group
that includes scorpions, spiders, ticks, and mites.
Nearly all ticks are blood-sucking parasites on the body
surfaces of reptiles or mammals.
 Parasitic mites live on or in a wide variety of vertebrates,
invertebrates, and plants.
The arachnid cephalothorax has six pairs of appendages.
 There are four pairs of walking legs.
 A pair of pedipalps function in sensing or feeding.
 The chelicerae usually function in feeding.
Spiders inject poison from glands on the chelicerae to
immobilize their prey and while chewing their prey, spill
digestive juices into the tissues and suck up the liquid meal.
In most spiders, gas exchange is carried out by book lungs.
 These are stacked plates contained in an internal chamber.
 The plates present an extensive surface area, enhancing
exchange of gases between the hemolymph and air.
A unique adaptation of many spiders is the ability to catch
flying insects in webs of silk.
 The silk protein is produced as a liquid by abdominal glands
and spun by spinnerets into fibers that solidify.
 Web designs are characteristic of each species.
 Silk fibers have other functions as egg covers, drop lines
for a rapid escape, and “gift wrapping” for nuptial gifts.
Millipedes and centipedes belong to the subphylum Myriapoda,
the myriapods.
 All living myriapods are terrestrial.
 Millipedes (class Diplopoda) have two pairs of walking legs on
each of their many trunk segments, formed by two fused
segments.
 They eat decaying leaves and plant matter.
 They may have been among the earliest land animals.
Centipedes (class Chilopoda) are terrestrial carnivores.
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 The head has a pair of antennae and three pairs of
appendages modified as mouth parts, including the jawlike
mandibles.
 Each segment in the trunk region has one pair of walking
legs.
 Centipedes have poison claws on the anteriormost trunk
segment that paralyze prey and aid in defense.
Insects and their relatives (subphylum Hexapoda) are more
species-rich than all other forms of life combined.
They live in almost every terrestrial habitat and in fresh
water, and flying insects fill the air.
 They are rare, but not absent, from the sea, where
crustaceans dominate.
The oldest insect fossils date back to the Devonian period,
about 416 million years ago.
 When insect flight evolved in the Carboniferous and Permian
periods, it sparked an explosion in insect varieties.
 Diversification of mouthparts for feeding on gymnosperms
and other Carboniferous plants also contributed to the
adaptive radiation of insects.
 In one widely held hypothesis, the radiation of flowering
plants triggered the greatest diversification of insects in
the Cretaceous and early Tertiary periods.
 However, new research suggests that insects diversified
first and, as pollinators and herbivores, may have caused
the angiosperm radiation.
Flight is one key to the great success of insects.
 Flying animals can escape many predators, find food and
mates, and disperse to new habitats faster than organisms
that must crawl on the ground.
Many insects have one or two pairs of wings that emerge from
the dorsal side of the thorax.
 Wings are extensions of the cuticle and are not true
appendages.
Several hypotheses have been proposed for the evolution of
wings.
 In one hypothesis, wings first evolved as extensions of the
cuticle that helped the insect absorb heat and were later
modified for flight.
 A second hypothesis argues that wings allowed animals to
glide from vegetation to the ground.
 Alternatively, wings may have served as gills in aquatic
insects.
 Still another hypothesis proposes that insect wings
functioned for swimming before they functioned for flight.
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 Insect wings are also very diverse.
 Dragonflies, among the first insects to fly, have two similar
pairs of wings.
 The wings of bees and wasps are hooked together and move
as a single pair.
 Butterfly wings operate similarly because the anterior wings
overlap the posterior wings.
 In beetles, the posterior wings function in flight, while the
anterior wings act as covers that protect the flight wings
when the beetle is on the ground or burrowing.
 The internal anatomy of an insect includes several complex
organ systems.
 In the complete digestive system, there are regionally
specialized organs with discrete functions.
 Metabolic wastes are removed from the hemolymph by
Malpighian tubules, outpockets of the digestive tract.
 Respiration is accomplished by a branched, chitin-lined
tracheal system that carries O2 from the spiracles directly
to the cells.
 The insect nervous system consists of a pair of ventral nerve
cords with several segmental ganglia.
 The two cords meet in the head, where the ganglia from
several anterior segments are fused into a cerebral ganglion
(brain).
 This structure is close to the antennae, eyes, and other
sense organs concentrated on the head.
 Metamorphosis is central to insect development.
 In incomplete metamorphosis (seen in grasshoppers and
some other orders), the young resemble adults but are
smaller and have different body proportions.
 Through a series of molts, the young look more and more
like adults until they reach full size.
 In complete metamorphosis, larval stages specialized for
eating and growing change morphology completely during the
pupal stage and emerge as adults.
 Reproduction in insects is usually sexual, with separate male
and female individuals.
 Coloration, sound, or odor bring together opposite sexes at
the appropriate time.
 In most species, sperm cells are deposited directly into the
female’s vagina at the time of copulation.
 In a few species, females pick up a sperm packet
deposited by a male.
 The females store sperm in the spermatheca, in some cases
holding enough sperm from a single mating to last a lifetime.
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 After mating, females lay their eggs on a food source
appropriate for the next generation.
Insects affect the lives of all other terrestrial organisms.
 Insects are important natural and agricultural pollinators.
 On the other hand, insects are carriers for many diseases,
including malaria and African sleeping sickness.
 Insects compete with humans for food, consuming crops
intended to feed and clothe human populations.
 Billions of dollars each year are spent by farmers on
pesticides to minimize losses to insects.
 In parts of Africa, insects claim about 75% of the crops.
While arachnids and insects thrive on land, most crustaceans
remain in marine and freshwater environments.
Crustaceans typically have biramous (branched) appendages
that are extensively specialized.
 Lobsters and crayfish have 19 pairs of appendages, adapted
to a variety of tasks.
 In addition to two pairs of antennae, crustaceans have
three or more pairs of mouthparts, including hard
mandibles.
 Walking legs are present on the thorax and other
appendages for swimming or reproduction are found on the
abdomen.
 Crustaceans can regenerate lost appendages during molting.
Small crustaceans exchange gases across thin areas of the
cuticle, but larger species have gills.
The circulatory system is open, with a heart pumping
hemolymph into short arteries and then into sinuses that bathe
the organs.
Nitrogenous wastes are excreted by diffusion through thin
areas of the cuticle, but glands regulate the salt balance of
the hemolymph.
Most crustaceans have separate sexes.
 In lobsters and crayfish, males use a specialized pair of
appendages to transfer sperm to the female’s reproductive
pore.
 Most aquatic species have several larval stages.
The isopods, with about 10,000 species, are one of the largest
groups of crustaceans.
 Most are small marine species, and some are abundant at
the bottom of deep oceans.
 Isopods also include the land-dwelling pill bugs, or wood lice,
that live underneath moist logs and leaves.
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 Decapods, including lobsters, crayfish, crabs, and shrimp, are
among the largest crustaceans.
 The cuticle is hardened with calcium carbonate.
 The exoskeleton over the cephalothorax forms a shield
called the carapace.
 While most decapods are marine, crayfish live in fresh
water and some tropical crabs are terrestrial as adults.
 Many small crustaceans are important members of marine and
freshwater plankton communities.
 Planktonic crustaceans include many species of copepods,
which are among the most numerous of all animals.
 Krill are shrimplike planktonic organisms that reach about 3
cm long.
 A major food source for baleen whales and other ocean
predators, they are now harvested extensively by humans
for food and agricultural fertilizer.
 Barnacles are primarily sessile crustaceans with parts of their
cuticle hardened by calcium carbonate.
 They anchor themselves to rocks, boat hulls, and pilings and
strain food from the water by extending their appendages.
 Their adhesive is as strong as any synthetic glue.
Concept 33.8 Echinoderms and chordates are deuterostomes
 At first glance, sea stars and other echinoderms would seem
to have little in common with the phylum Chordata, which
includes the vertebrates.
 However, these animals share the deuterostome
characteristics of radial cleavage, type of development of the
coelom from the archenteron, and formation of the anus from
the blastopore.
 Molecular systematics has reinforced the Deuterostomia as a
clade of bilaterian animals.
Phylum Echinodermata: Echinoderms have a water vascular
system and secondary radial symmetry.
 Sea stars and most other echinoderms are sessile or slowmoving marine animals.
 A thin skin covers an endoskeleton of hard calcareous plates.
 Most echinoderms are prickly from skeletal bumps and
spines that have various functions.
 Unique to echinoderms is the water vascular system, a
network of hydraulic canals branching into extensions called
tube feet.
 These function in locomotion, feeding, and gas exchange.
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 Sexual reproduction in echinoderms usually involves the
release of gametes by separate males and females into the
seawater.
 The internal and external parts of the animal radiate from the
center, often as five spokes.
 However, the radial anatomy of adult echinoderms is a
secondary adaptation, as echinoderm larvae have bilateral
symmetry.
 The symmetry of adult echinoderms is not perfectly radial.
 Living echinoderms are divided into six classes: Asteroidea
(sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins
and sand dollars), Crinoidea (sea lilies and feather stars),
Holothuroidea (sea cucumbers), and Concentricycloidea (sea
daisies).
 Sea stars have multiple arms radiating from a central disk.
 The undersides of the arms have rows of tube feet.
 Each can act like a suction disk that is controlled by
hydraulic and muscular action.
 Sea stars use the tube feet to grasp the substrate, to creep
slowly over the surface, or to capture prey.
 When feeding on closed bivalves, the sea star grasps the
bivalve tightly and everts its stomach through its mouth and
into the narrow opening between the shells of the bivalve.
 Enzymes from the sea star’s digestive organs then begin to
digest the soft body of the bivalve inside its own shell.
 Sea stars and some other echinoderms can regenerate lost
arms and, in a few cases, even regrow an entire body from a
single arm.
 Brittle stars have a distinct central disk and long, flexible
arms.
 Their tube feet lack suckers.
 They move by a serpentine lashing of their arms.
 Some species are suspension feeders, and others are
scavengers or predators.
 Sea urchins and sand dollars have no arms, but they do have
five rows of tube feet that are used for locomotion.
 Sea urchins can also move by pivoting their long spines.
 The mouth of an urchin is ringed by complex jawlike
structures adapted for eating seaweed and other foods.
 Sea urchins are roughly spherical, while sand dollars are
flattened and disk-shaped.
 Sea lilies are attached to the substratum by stalks, and
feather stars crawl using their long, flexible arms.
 Both use their arms for suspension feeding.
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 The arms circle the mouth, which is directed upward, away
from the substrate.
 Crinoids are an ancient class with very conservative
evolution.
 Fossilized sea lilies from 500 million years ago could pass
for modern members of the class.
 Sea cucumbers do not look much like other echinoderms.
 They lack spines, the endoskeleton is much reduced, and the
oral-aboral axis is elongated.
 However, they do have five rows of tube feet, like other
echinoderms, and other shared features.
 Some tube feet around the mouth function as feeding
tentacles for suspension feeding or deposit feeding
 Sea daisies were discovered in 1986, and only two species are
known.
 Their armless bodies are disk-shaped with five-fold
symmetry.
 They are less than a centimeter in diameter.
 Sea daisies absorb nutrients through the membrane
surrounding their body.
 Some taxonomists consider sea daisies to be highly derived
sea stars.
Phylum Chordata: The chordates include two invertebrate
subphyla and all vertebrates.
 The phylum to which we belong consists of two subphyla of
invertebrate animals plus the hagfishes and vertebrates.
 Both groups of deuterostomes, the echinoderms and
chordates, have existed as distinct phyla for at least half a
billion years.
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